An extrusion head is known, from British Patent Specification No. 2131734A, which is fed by a plurality of extrusion devices disposed one above the other. The head comprises a fixed central portion and upper and lower portions which are pivotable towards and away from the central portion. Between each of the upper and lower portions and the central portion, insert members are provided which, together with an adjacent planar surface of respective ones of the head portions, define flow channels for the materials extruded from the extrusion devices. It is also known to provide a front template in the head which has flow channel sections formed therein and an end template which determines the external configuration of the profile being extruded.
Such an arrangement can be used for producing tread strip portions for motor vehicle tyres. In such a case, the profile is made from three different types of mixture. Thus, the mixture used to produce the tread surface must be highly wear-resistant, that used to form the lateral portions must be very resilient and the lower or base portion is formed from a mixture which is softer than the tread surface mixture.
These three mixtures, the viscosities of which differ considerably from one another must be brought together in the extrusion head and combined to form a single tread strip profile. However, the mixtures must have approximately the same flow rates in the flow channels.
In the arrangement disclosed in British Patent Specification No. 2131734A, it is necessary that the upper and lower portions are pivotable so that the flow channels can be cleaned when the mixture flowing therethrough is to be changed or if different flow channel insert members are to be inserted because the dimensions of the profile being produced are to be changed. The insert members disposed in the head are each formed in one piece with the flow channels being formed in their upper and/or lower surfaces. If it is desired to extrude a different size of tread strip portion, the end and, if necessary, the front template are changed. If the tread strip sizes differ significantly from one another, the insert members are also changed.
In practice, there may be eight to ten changes of the tread strip mixture in each working shift so as to permit, for example, both summer and winter tyres of different sizes to be produced during the shift. When a new profile is to be produced, it is usually the rubber mixture for the tread strip portion which is changed, whereas the rubber mixtures for the lateral strips and lower or base layer are only changed infrequently. To permit the mixture for the tread strip to be changed, the extrusion head must be upwardly pivoted.
As a result of such upward pivotal movement, the material in the flow channels becomes deformed and, in consequence, has to be removed. In addition, air enters the flow channels through which rubber mixtures which are not to be changed are flowing. This causes the formation of air bubbles therein and deviations in the dimensions of the profile when the apparatus is restarted. Accordingly, unusable waste material is produced until the air has been expressed from the inserts, the front template and the end template.
Accordingly, all of the extrusion devices have to be emtied and refilled, even though only one of the mixtures has been changed. This means that a considerable amount of time and material is wasted.